Well known, proposed apparatus of various types, is a liquid separation apparatus of spiral type, employing reverse osmosis, which can be operated with a high efficiency of liquid separation. The spiral type apparatus comprises at least two semipermeable membrane sheets which form an envelope having an open end, and one kind of spacing layer for a first passage for a permeated solution and another kind of spacing layer for second passage for a feed solution or a nonpermeated solution. The membrane envelope and the two kinds of spacing layers are spirally wound about a hollow mandrel having at least one hole. The first passage is formed inside of the membrane envelope and communicates with the interior of the hollow mandrel through the hole, while the second passage is formed outside of the membrane envelope.
The specification of U.S. Pat. No. 3,367,504 discloses one kind of spiral type apparatus, wherein the second passage for the feed solution is arranged so that the feed solution is forced to flow from the spiral edges of the membrane sheets and pass through the second passage in a direction parallel to the axis of the mandrel; while the permeated liquid from the feed solution passes through the membrane sheets into the first passage by reverse osmosis and flows spirally through the first passage, and is discharged from the first passage into the hollow mandrel. This kind of apparatus may be referred to as axial-spiral type apparatus.
The other kind of spiral type apparatus is disclosed in the specification of U.S. Pat. No. 3,933,645 and may be referred to as spiral-spiral type apparatus. The spiral-spiral type apparatus is designed so that the second passage for the feed solution has an inlet opening elongated in the axial direction at the outer edges of the entire lengths of the sheets and an axial outlet opening at the spiral edges of the sheets in the vicinity of the surface of the mandrel. The first and second passages are closed at the opposite spiral edges of the sheets over most of the spiral lengths, except for the partial lengths where the axial outlet opening is formed. In this apparatus, the feed solution, under increased pressure, passes spirally in a direction perpendicular to the axis through the second passage in contact with the sheets on a plane perpendicular to the axis, while a solution having a concentrated solvent permeates from the feed solution by reverse osmosis through the sheets into the first passage. The resultant solution having a concentrated solute, that is, the nonpermeated solution, flows spirally in a direction perpendicular to the axis through the spiral second passage and, then, is forced to change its course axially in the vicinity of the surface of mandrel to flow that surface through the second passage, and to flow out of the axial outlet of the second passage, while the perpeated solution is forced to flow into the interior of the mandrel. The formerly mentioned axial-spiral type apparatus has the disadvantage that it exhibits a relatively low performance with a low rejection, which is a function of the module, and with a low recovering ratio, which ratio is defined by the ratio of a volume of the permeated solution to the feed solution. This is caused by inherent problems in that the sectional area of the feed solution passage is apt to vary considerably owing to the fluid pressure of the feed solution or the nonpermeated solution and, thus, the flow velocity varies on a plane perpendicular to the axial direction. This results in an unequally distributed fluid flow in the second passage for the feed solution, and produces the unfavorable phenomenon of polarized concentration. Such a phenomenon causes a decrease in rejection which is calculated by the following formula. ##EQU1## wherein: R: rejection in percentage;
C.sub.F : concentration of solute in nonpermeated solution after discharge; PA1 C.sub.S : concentration of solute in permeated solution after discharge. PA1 C.sub.o : concentration of solute in feed solution
The spiral-spiral type apparatus is advantageous in its performance, compared with the axial-spiral type apparatus. This is because such unequal distribution of the flow velocity as in the axial-spiral type apparatus does not occur in the spiral-spiral type apparatus. However, the spiral-spiral type apparatus has a disadvantage in that a considerably high fluid pressure loss or pressure drop is produced in the second passage, that is the feed solution passage, compared with the axial-spiral type apparatus. Such high fluid pressure loss in the feed solution passage presents various problems in the running of the apparatus. For example, one problem is that the operational fluid pressure of the feed solution must be increased by a pressure corresponding to the fluid pressure loss, to ensure continuation of the feed solution supply to the second passage for the feed solution. This results in a high consumption of electric power by a feeding pump. Further, it should be noted that the apparatus, in general, comprises: a cylindrical chamber where the feed solution is fed by the pump, and; a module, mounted in the chamber, including the mandrel, the spacing layers and membrane sheets spirally wound about the mandrel. In this connection, the membrane sheets are depressed against the spacing layers and the spacing layers are compressed by the highly pressurized feed solution enclosing the module in the chamber. This leads to decreased performance of the membrane sheets exhibitting the reverse osmosis, and results in difficulty in removal of the materials or impurities having adhered from the feed solution flown into the spacing layer to the spacing layer, as well as the inner surfaces of the membrane sheets. Such disadvantageous problems involved in the spiral-spiral type apparatus as mentioned the above are also presented in a spiral-axial type apparatus, wherein a feed solution is forced to flow spirally, while a permeated solution is forced flow, in a direction parallel to the axis, just like the feed solution in the spiral-spiral type apparatus.